There are two basic types of hot runner molds, namely, insulated runner molds and true hot runner molds. In an insulated runner mold, an unheated runner extends between a machine nozzle and a mold cavity. In use, molten plastic ("melt") flows from the nozzle through the runner into a space defined between respective mold faces of a mold cavity and a mold core to form a part. During use, a portion of the plastic solidifies adjacent the runner walls and acts as an insulator for a molten core of plastic, hence the term "insulated runner".
Immediately adjacent the mold face of the mold cavity is an aperture referred to as a "gate" through which melt exits the insulated runner to form the part. It is of course necessary to interrupt flow through the gate for mold opening and stripping portions of an injection molding cycle. In insulated runner systems, melt is generally allowed to "freeze" in the gate prior to mold opening. Early systems would re-open the gate by pushing the frozen "slug" of plastic into the next part by incoming melt which would also melt the slug.
More recently developed systems use a nozzle heater, generally referred to by persons skilled in such devices as a "torpedo" to better control melt temperature adjacent the gate. The torpedo is situated in an area referred to as a "runner drop" at a gate end of the insulated runner. The use of a heated torpedo allows longer cycle times by extending the critical time in which freezing could occur which would require "restarting" of the molding process.
A drawback to insulated runner systems is the presence of a "vestige" on the part in the vicinity of the gate and "stringing" which result from melt in the gate adhering to the part during mold opening. In order to produce more acceptable gate vestiges, the gate should ideally be closeable other than by a slug of plastic after injection and before mold separation. Various arrangements have been devised to date. Each has its drawbacks.
U.S. Pat. No. 3,159,878 to Scott teaches an insulated runner mold that uses a moveable core plug in the mold to keep the gate closed until sufficient pressure has built up inside the runner to overcome the closing force of the plug thereby "blasting open" the gate to rapidly fill a thin wall container mold.
U.S. Pat. No. 5,037,598 to Akselrud teaches a moveable heated torpedo in an insulated runner drop that acts as a gate valve. However, to open the gate the torpedo is advanced toward the gate to melt the cold slug therein and thereafter the torpedo is retracted to allow melt to flow. When the mold is filled the gate remains open and freezes off, it is not closed by advancing the torpedo.
Both these approaches produce gate vestiges on the part the size of which are a function of gate diameter. The larger the diameter the larger the vestige. Large gate diameters promote good flow and rapid filling of the mold cavity when high viscosity, or filled resins are being processed. In contrast smaller gate diameters, which give smaller vestiges, make filling slower and more difficult for filled resins and for making thin walled parts.
It is an object of the present invention to provide a valve gated insulated runner to enable maximization of gate opening while substantially eliminating the gate vestige to produce a substantially blemish free part in any application.